Method for controlling laser power of an optical pickup unit

ABSTRACT

A method for controlling laser power of an optical pickup unit (OPU) includes: providing a first relationship between the laser power and a driving parameter, wherein the driving parameter is utilized for driving a laser diode (LD) of the OPU, and the first relationship corresponds to a first temperature; utilizing a temperature-related model to convert the first relationship into a second relationship between the laser power and the driving parameter, wherein the second relationship corresponds to a second temperature; and storing the first relationship for being utilized at the first temperature, and storing the second relationship for being utilized at the second temperature.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 60/991,185, which was filed on Nov. 29, 2007, and entitled “AUTO POWER CALIBRATION STRUCTURAL”.

BACKGROUND

The present invention relates to power calibration of an optical pickup unit (OPU) with respect to temperature during a mass production phase of an optical disc drive, and more particularly, to a method for controlling laser power of an OPU.

Regarding the control over an OPU of an optical disc drive in the related art, a conventional automatic power calibration (APC) circuit can be utilized for controlling the laser power of a laser diode (LD) during a normal operation of the optical disc drive, e.g. a reading/writing operation. When the conventional APC circuit reaches a steady state during the normal operation mentioned above, the laser power corresponds to a target command that is sent to the conventional APC circuit.

FIG. 1 illustrates a relationship between the laser power (labeled “Power” as shown on the vertical axis) and a driving parameter such as an LD driving voltage (labeled “Voltage” as shown on the horizontal axis) according to the related art. The LD driving voltage is utilized for controlling an LD driving current of the LD, where the LD driving voltage corresponds to the target command when the steady state mentioned above is reached. As shown in FIG. 1, a dashed line aligned to the curve corresponding to the linear region has an offset Vth with regard to the horizontal axis.

It is a goal for the conventional APC circuit to control the laser power to be a specific power value corresponding to the target command, in order that the laser power varies in accordance with the target command. Thus, how to prepare a precise relationship between the laser power and the target command during a mass production phase of the optical disc drive has become an important issue

A conventional method for deriving the relationship between the laser power and the target command typically comprises measuring the laser power by utilizing a power meter, and collecting data sets of the laser power and the target command. However, the cost of the power meter is high, and the corresponding tooling and labor costs of a power calibration station for implementing this method are also required. In addition, these costs will be multiplied according to the number of production lines. Furthermore, other issues such as the differences between respective power calibration stations may arise.

According to the related art, an OPU vendor may design a front-end photo diode (PD) in an OPU, and the system manufacturers (e.g. an optical disc drive manufacturer) may use the front-end PD as a replacement for the power meter. The measurement result from the front-end PD is outputted through a front-end PD output (FPDO), and can be referred to as the FPDO value. As the OPU vendor typically provides a few data points for stating the relationship between the laser power and the FPDO value, interpolation operations are required for deriving the laser power corresponding to other data points on a predicted curve passing through the few data points mentioned above. As a result, the whole process of deriving a precise relationship between the laser power and the target command is slowed down due to the interpolation operations.

Thus, no matter whether the calibration in the power calibration station is implemented by utilizing the power meter or the FPDO, the corresponding costs such as time, tooling and/or labor costs are required. Moreover, there is little awareness of the inaccuracy due to temperature variation during the normal operation. As a result, the calibration is often performed at only an arbitrary temperature.

Even if the inaccuracy due to the temperature variation during the normal operation is noticed, performing the calibration in the power calibration station with respect to different values of temperature will be cost-ineffective for most system manufacturers utilizing the related art methods.

Therefore, the control over the laser power will certainly be inaccurate in a normal operation when the temperature varies. A novel method is therefore required for solving the related art problems, such as the inaccuracy due to the temperature variation, and the tradeoff between the costs and the performance.

SUMMARY

It is therefore an objective of the claimed invention to provide a method for controlling laser power of an optical pickup unit (OPU), in order to solve the above-mentioned problems.

An exemplary embodiment of a method for controlling laser power of an OPU comprises: providing a first relationship between the laser power and a driving parameter, wherein the driving parameter is utilized for driving a laser diode (LD) of the OPU, and the first relationship corresponds to a first temperature; utilizing a temperature-related model to convert the first relationship into a second relationship between the laser power and the driving parameter, wherein the second relationship corresponds to a second temperature; and storing the first relationship for being utilized at the first temperature, and storing the second relationship for being utilized at the second temperature.

An exemplary embodiment of a method for controlling laser power of an OPU comprises: providing a first relationship between the laser power and a driving parameter, wherein the driving parameter is utilized for driving an LD of the OPU, and the first relationship corresponds to a first temperature; providing a second relationship between the laser power and the driving parameter, wherein the second relationship corresponds to a second temperature; and storing the first relationship for being utilized at the first temperature, and storing the second relationship for being utilized at the second temperature.

These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a relationship between laser power of an optical pickup unit (OPU) and a driving parameter according to the related art, where the driving parameter is utilized for driving a laser diode (LD) of the OPU and controlling the laser power emitted from the LD.

FIG. 2 is a flowchart of a method for controlling laser power of an OPU according to a first embodiment of the present invention.

FIG. 3 and FIG. 4 illustrate examples of different temperature-related models that can be utilized by the method shown in FIG. 2.

FIG. 5 is an exemplary functional diagram illustrating various gains applied to different channels regarding an LD driver (LDD) for driving an LD in the OPU according to a variation of the first embodiment.

DETAILED DESCRIPTION

Certain terms are used throughout the following description and claims, which refer to particular components. As one skilled in the art will appreciate, electronic equipment manufacturers may refer to a component by different names. This document does not intend to distinguish between components that differ in name but not in function. In the following description and in the claims, the terms “include” and “comprise” are used in an open-ended fashion, and thus should be interpreted to mean “include, but not limited to . . . ”. Also, the term “couple” is intended to mean either an indirect or direct electrical connection. Accordingly, if one device is coupled to another device, that connection may be through a direct electrical connection, or through an indirect electrical connection via other devices and connections.

Please refer to FIG. 2, FIG. 3 and FIG. 4. FIG. 2 is a flowchart of a method for controlling laser power of an optical pickup unit (OPU) according to a first embodiment of the present invention, where the method can be utilized for obtaining precise control over the laser power of the OPU. FIG. 3 and FIG. 4 illustrate examples of different temperature-related models that can be utilized by the method shown in FIG. 2. The method for controlling the laser power of the OPU is described as follows.

In Step 912, a first relationship between the laser power and a driving parameter is provided, where the driving parameter is utilized for driving a laser diode (LD) of the OPU to control the laser power, and the first relationship corresponds to a first temperature such as room temperature. The relationships between the laser power (which can be simply referred to as power) and the driving parameter (e.g. voltage) are different according to temperature values. For example, the first relationship can be depicted as one of the curves labeled “Middle Temperature” in the temperature-related models respectively shown in FIG. 3 and FIG. 4. Moreover, the slopes and the Vth of the curves with different temperatures are different.

According to this embodiment, the vertical axis (labeled “Power”) shown in FIG. 3 and FIG. 4 depicts the laser power mentioned above, and the horizontal axis (labeled “Voltage”) shown in FIG. 3 and FIG. 4 depicts the driving parameter mentioned above. Although the driving parameter in either of the temperature-related models of this embodiment represents an LD driving voltage (e.g. the “Voltage” shown in FIG. 3 or FIG. 4) for controlling an LD driving current of the LD, this is only for illustrative purposes and is not meant to be a limitation of the present invention. According to a variation of this embodiment, the driving parameter represents the LD driving current of the LD.

In practice, the first relationship is typically derived by measuring the laser power and the driving parameter at the first temperature. According to this embodiment, a target command carrying a specific value can be first applied to an automatic power calibration (APC) circuit for controlling the laser power. When the APC circuit reaches a steady state at the first temperature, the laser power and the driving parameter are measured in order to derive the first relationship. Typically, when partial information of the first relationship is given (e.g. a slope or an offset of a curve of the first relationship), only measuring a single data point comprising a specific value of the laser power and a specific value of the driving parameter may be enough for deriving the first relationship, where such a data point can be utilized for depicting a curve of the first relationship with the horizontal and the vertical axes respectively representing the driving parameter and the laser power.

For example, the target command carrying a specific value V₁ can be first applied to the APC circuit. When the APC circuit reaches a steady state at the first temperature, the laser power and the driving parameter are measured to derive the single data point P₁. Thus, the single data point P₁ can be utilized for deriving the first relationship. In this embodiment, the single data point P₁ is illustrated on the curves labeled “Middle Temperature” in the temperature-related models respectively shown in FIG. 3 and FIG. 4. This is for illustrative purposes only, and is not meant to be a limitation of the present invention.

According to a variation of this embodiment, the single data point P₁ is illustrated on the curves labeled “Low Temperature” in the temperature-related models respectively shown in FIG. 3 and FIG. 4. According to another variation of this embodiment, the single data point P₁ is illustrated on the curves labeled “High Temperature” in the temperature-related models respectively shown in FIG. 3 and FIG. 4. According to another variation of this embodiment, more than one data point can be illustrated on one of the curves shown in FIG. 3 and FIG. 4.

Sometimes, measuring two data points may be required, where the two data points are derived according to the same method as that for deriving the single data point with regard to different values of the target command, respectively. For example, the target command carrying a first value V₁₋₁ can be first applied to the APC circuit. When the APC circuit reaches a steady state at the first temperature, the laser power and the driving parameter (such as voltage) are measured to derive a first data point P₁₋₁. Afterward, the target command carrying a second value V₁₋₂ can be applied to the APC circuit. When the APC circuit reaches a steady state at the first temperature, the laser power and the driving parameter (such as voltage) are measured to derive a second data point P₁₋₂. Thus, the first data point P₁₋₁ and the second data point P₁₋₂ can be utilized for deriving the first relationship.

In Step 914, a temperature-related model is utilized, such as one of the two temperature-related models shown in FIG. 3 and FIG. 4, to convert the first relationship into a second relationship between the laser power and the driving parameter, where the second relationship corresponds to a second temperature, and the second temperature is different from the first temperature.

As shown in FIG. 3, the temperature-related model corresponds to curves having respective slopes with respect to different temperatures. In the situation where the temperature-related model shown in FIG. 3 is utilized, extension lines of the linear portions of these curves (labeled “Low Temperature”, “Middle Temperature”, and “High Temperature”, respectively) are substantially concurrent lines. In addition, as shown in FIG. 4, the temperature-related model corresponds to parallel curves with respect to different temperatures. In the situation where the temperature-related model shown in FIG. 4 is utilized, the offset Vth with regard to the horizontal axis varies when the middle curve (labeled “Middle Temperature”) shifts to the left or to the right.

According to this embodiment, no matter whether the temperature-related model shown in FIG. 3 or the temperature-related model shown in FIG. 4 is utilized, measuring at least one data point comprising a specific value of the laser power and a specific value of the driving parameter at the second temperature may be required for deriving sufficient information of the temperature-related model. In this embodiment, the relationship conversion in Step 914 may require a single data point P₂ to be measured at the second temperature in order to derive the second relationship such as that depicted by the left curve (labeled “Low Temperature” in FIG. 3 or FIG. 4) or that depicted by the right curve (labeled “High Temperature” in FIG. 3 or FIG. 4).

The method for deriving the single data point P₂ at the second temperature is similar to that for deriving the single data point P₁ at the first temperature except for the ambient temperature during the measurement. For example, the target command carrying a specific value V₂ can be first applied to the APC circuit. When the APC circuit reaches a steady state at the second temperature, the laser power and the driving parameter are measured to derive the single data point P₂. Thus, the data point P₂ can be utilized for deriving the sufficient information of the temperature-related model.

According to a variation of this embodiment, measuring two data points at the second temperature may be required in order to derive sufficient information of the temperature-related model. In this variation, after deriving the sufficient information of the temperature-related model, whether the temperature-related model shown in FIG. 3 or the temperature-related model shown in FIG. 4 should be utilized can be determined. The method for deriving the two data points at the second temperature is similar to that for deriving the two data points at the first temperature except for the ambient temperature during the measurement.

For example, the target command carrying a first value V₂₋₁ can be first applied to the APC circuit. When the APC circuit reaches a steady state at the second temperature, the laser power and the driving parameter are measured to derive a first data point P₂₋₁. Afterward, the target command carrying a second value V₂₋₂ can be applied to the APC circuit. When the APC circuit reaches a steady state at the second temperature, the laser power and the driving parameter are measured to derive a second data point P₂₋₂. Thus, the first data point P₂₋₁ and the second data point P₂₋₂ can be utilized for deriving the sufficient information of the temperature-related model.

In Step 916, the first relationship for being utilized at the first temperature is stored, and the second relationship for being utilized at the second temperature is stored. In practice, the various representatives of the first relationship and the second relationship can be stored in a non-volatile memory such as a Flash memory. For example, the representatives can be curve coefficients of the curves representing the first relationship and the second relationship, respectively. In addition, the representatives in another example can be one or more data points for each of the first relationship or the second relationship. Additionally, the representatives in another example can be one or more curve coefficients together with one or more data points.

FIG. 5 is an exemplary functional diagram illustrating various gains applied to different channels regarding an LD driver (LDD) 20 for driving the LD in the OPU according to a variation of the first embodiment, where the functional blocks 10-1, 10-2, . . . , and 10-N (labeled “Gain(1)”, “Gain(2)”, . . . , and “Gain(N)”) represent channel gain functions of the channels 1 to N, respectively. Although the channels 1 to N are illustrated as several paths, this is only for illustrative purposes and is not meant to be a limitation of the present invention. In practice, the channel gain functions Gain(1), Gain(2), . . . , and Gain(N) are typically implemented with the same hardware circuit to be integrated as a variable gain amplifier (VGA) 10 in the APC circuit mentioned above.

According to this variation, a method for transforming a voltage difference ΔV_(X) ^((T1, T2)) corresponding to a channel X (e.g. one of the channels 1 to N) into a voltage difference ΔV_(Y) ^((T1, T2)) corresponding to a channel Y (e.g. another of the channels 1 to N) is further provided, in order to make sure the method mentioned above can be widely applied. As a result, some special situations can be covered. The voltage differences ΔV_(X) ^((T1, T2)) and ΔV_(Y) ^((T1, T2)) mentioned above respectively have two indexes T1 and T2 representing two different values of temperature T, and are typically defined as follows:

ΔV _(X) ^((T1, T2))=Volt_(X) ^((T2))−Volt_(X) ^((T1)); and

ΔV _(Y) ^((T1, T2))=Volt_(Y) ^((T2))−Volt_(Y) ^((T1));

where Volt_(X) ^((T)) represents a voltage value of the LD driving voltage in the channel X at temperature T, and Volt_(Y) ^((T)) represents a voltage value of the LD driving voltage in the channel Y at temperature T.

According to this variation, the voltage difference ΔV_(Y) ^((T1, T2)) can be derived by the following equation:

ΔV _(Y) ^((T1, T2))=(Gain(X)/Gain(Y))*ΔV _(X) ^((T1, T2));

where Gain(X) and Gain(Y) represent the channel gain functions of the channels X and Y, respectively.

For example, the voltage difference ΔV_(X) ^((T1, T2)) is derived from the method shown in FIG. 2 with T1 and T2 respectively representing the first and the second temperatures mentioned above. When the voltage value Volt_(Y) ^((T1)) is known, the voltage value Volt_(Y) ^((T2)) can be derived as follows:

$\begin{matrix} {{Volt}_{Y}^{({T\; 2})} = {{\Delta \; V_{Y}^{({{T\; 1},{T\; 2}})}} + {Volt}_{Y}^{({T\; 1})}}} \\ {{= {{\left( {{{Gain}(X)}/{{Gain}(Y)}} \right)*\Delta \; V_{X}^{({{T\; 1},{T\; 2}})}} + {Volt}_{Y}^{({T\; 1})}}};} \end{matrix}$

where the above equation can be utilized for deriving an additional relationship between the laser power and the driving parameter in the channel Y.

As the channels X and Y mentioned above may represent any two of the channels 1 to N, the relationships between the channel gain functions Gain(1), Gain(2), . . . , and Gain(N) and the voltage differences respectively corresponding to the channels can be expressed, in general, by utilizing the following equation:

ΔV _(channel 1)*Gain(1)=ΔV _(channel 2)*Gain(2)= . . . =ΔV _(channel N)*Gain(N);

where ΔV_(channel 1), ΔV_(channel 2), . . . , and ΔV_(channel N) represent the voltage differences corresponding to the channels 1 to N, respectively.

In another variation of the first embodiment, the voltage values Volt_(X) ^((T)) and Volt_(Y) ^((T)) of the LD driving voltage can be replaced with corresponding current values Curr_(X) ^((T)) and Curr_(Y) ^((T)) of the LD driving current mentioned above, and the voltage differences ΔV_(X) ^((T1, T2)) and ΔV_(Y) ^((T1, T2)) can be replaced with corresponding current differences ΔC_(X) ^((T1, T2) and ΔC) _(Y) ^((T1, T2)), respectively.

As a result of applying the method mentioned above, the present invention indeed provides proper control over laser power in the normal operation when the temperature varies. In contrast to the related art, by performing the relationship conversion and measuring of a few data points as disclosed in the embodiment(s) and/or variations mentioned above, the present invention method greatly saves the costs required for the calibration in the power calibration station, and therefore solves the tradeoff between the costs and the performance.

It is another advantage of the present invention that, when the laser power control should be implemented with respect to a large number of values of temperature, the relationships corresponding to these values of temperature can be efficiently derived according to the embodiments and/or variations mentioned above.

Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims. 

1. A method for controlling laser power of an optical pickup unit (OPU), the method comprising: providing a first relationship between the laser power and a driving parameter, wherein the driving parameter is utilized for driving a laser diode (LD) of the OPU, and the first relationship corresponds to a first temperature; utilizing a temperature-related model to convert the first relationship into a second relationship between the laser power and the driving parameter, wherein the second relationship corresponds to a second temperature; and storing the first relationship for being utilized at the first temperature, and storing the second relationship for being utilized at the second temperature.
 2. The method of claim 1, wherein the driving parameter represents an LD driving voltage for controlling an LD driving current of the LD.
 3. The method of claim 1, wherein the driving parameter represents an LD driving current of the LD.
 4. The method of claim 1, wherein the step of providing the first relationship between the laser power and the driving parameter further comprises: applying a target command carrying a specific value to an automatic power calibration (APC) circuit for controlling the laser power; and when the APC circuit reaches a steady state at the first temperature, measuring the laser power and the driving parameter to derive the first relationship.
 5. The method of claim 1, wherein the step of providing the first relationship between the laser power and the driving parameter further comprises: applying a target command carrying a first value to an automatic power calibration (APC) circuit for controlling the laser power, and when the APC circuit reaches a steady state at the first temperature, measuring the laser power and the driving parameter to derive a first data point; applying a target command carrying a second value to the APC circuit, and when the APC circuit reaches a steady state at the first temperature, measuring the laser power and the driving parameter to derive a second data point; and utilizing the first and second data points to derive the first relationship.
 6. The method of claim 1, wherein the temperature-related model corresponds to curves having respective slopes with respect to different temperatures.
 7. The method of claim 1, wherein the temperature-related model corresponds to parallel curves with respect to different temperatures.
 8. The method of claim 1, further comprising: measuring the laser power and the driving parameter at the second temperature to derive information of the temperature-related model.
 9. The method of claim 8, wherein the step of measuring the laser power and the driving parameter at the second temperature to derive the information of the temperature-related model further comprises: applying a target command carrying a specific value to an automatic power calibration (APC) circuit for controlling the laser power; and when the APC circuit reaches a steady state at the second temperature, measuring the laser power and the driving parameter to derive the information of the temperature-related model.
 10. The method of claim 8, further comprising: applying a target command carrying a first value to an automatic power calibration (APC) circuit for controlling the laser power, and when the APC circuit reaches a steady state at the second temperature, measuring the laser power and the driving parameter to derive a first data point; applying a target command carrying a second value to the APC circuit, and when the APC circuit reaches a steady state at the second temperature, measuring the laser power and the driving parameter to derive a second data point; and utilizing the first and second data points to derive the information of the temperature-related model.
 11. The method of claim 1, wherein the first and second relationships correspond to a first channel; and the method further comprises: utilizing the first and second relationships to derive an additional relationship between the laser power and the driving parameter in a second channel.
 12. The method of claim 11, wherein the driving parameter represents an LD driving voltage; and the step of utilizing the first and second relationships to derive the additional relationship between the laser power and the driving parameter in the second channel further comprises: transforming a first voltage difference into a second voltage difference; wherein the first voltage difference represents a difference between different voltage values of the LD driving voltage in the first channel at the first and the second temperatures, respectively; wherein the second voltage difference represents a difference between different voltage values of the LD driving voltage in the second channel at the first and the second temperatures, respectively.
 13. The method of claim 11, wherein the driving parameter represents an LD driving current of the LD; and the step of utilizing the first and second relationships to derive the additional relationship between the laser power and the driving parameter in the second channel further comprises: transforming a first current difference into a second current difference; wherein the first current difference represents a difference between different current values of the LD driving current in the first channel at the first and the second temperatures, respectively; wherein the second current difference represents a difference between different current values of the LD driving current in the second channel at the first and the second temperatures, respectively.
 14. A method for controlling laser power of an optical pickup unit (OPU), the method comprising: providing a first relationship between the laser power and a driving parameter, wherein the driving parameter is utilized for driving a laser diode (LD) of the OPU, and the first relationship corresponds to a first temperature; providing a second relationship between the laser power and the driving parameter, wherein the second relationship corresponds to a second temperature; and storing the first relationship for being utilized at the first temperature, and storing the second relationship for being utilized at the second temperature.
 15. The method of claim 14, wherein the driving parameter represents an LD driving voltage for controlling an LD driving current of the LD.
 16. The method of claim 14, wherein the driving parameter represents an LD driving current of the LD.
 17. The method of claim 14, wherein the step of providing the first relationship between the laser power and the driving parameter further comprises: applying a target command carrying a specific value to an automatic power calibration (APC) circuit for controlling the laser power; and when the APC circuit reaches a steady state at the first temperature, measuring the laser power and the driving parameter to derive the first relationship.
 18. The method of claim 14, wherein the step of providing the first relationship between the laser power and the driving parameter further comprises: applying a target command carrying a first value to an automatic power calibration (APC) circuit for controlling the laser power, and when the APC circuit reaches a steady state at the first temperature, measuring the laser power and the driving parameter to derive a first data point; applying a target command carrying a second value to the APC circuit, and when the APC circuit reaches a steady state at the first temperature, measuring the laser power and the driving parameter to derive a second data point; and utilizing the first and second data points to derive the first relationship.
 19. The method of claim 14, wherein the step of providing the second relationship between the laser power and the driving parameter further comprises: applying a target command carrying a specific value to an automatic power calibration (APC) circuit for controlling the laser power; and when the APC circuit reaches a steady state at the second temperature, measuring the laser power and the driving parameter to derive the second relationship.
 20. The method of claim 14, wherein the step of providing the second relationship between the laser power and the driving parameter further comprises: applying a target command carrying a first value to an automatic power calibration (APC) circuit for controlling the laser power, and when the APC circuit reaches a steady state at the second temperature, measuring the laser power and the driving parameter to derive a first data point; applying a target command carrying a second value to the APC circuit, and when the APC circuit reaches a steady state at the second temperature, measuring the laser power and the driving parameter to derive a second data point; and utilizing the first and second data points to derive the second relationship. 